The present invention relates to a superregenerative detector and in particular to a superregenerative-detector having a surface acoustic wave device in the feed back loop of the oscillator to cause oscillations.
The superregenerative detector is a regenerative circuit in which an ordinary oscillator is automatically switched between an oscillating and a nonoscillating condition at a low radio frequency rate. The switching frequency is called the "quenching" frequency. In general, the quenching frequency is increased as the carrier modulation frequency becomes greater. With proper adjustment, oscillations will build up in the detector near the positive peaks of each cycle of the low frequency "quench" voltage and will die out when the voltage is negative or too low to provide oscillating conditions. The signal voltage that is to be detected is connected to a feedback loop which is connected to a transistor amplifier to cause oscillation.
In the absence of an applied signal, the oscillations that build up during each cycle of the quench voltage start with an initial amplitude determined by the noise voltages in the input circuit and reach a final value corresponding to the equilibrium value for the oscillator. These oscillations then die out as the "quench" voltage becomes small and then goes negative or too low to provide oscillating conditions. If an RF carrier is superimposed upon the system and is larger in magnitude than the thermal agitation noises in the tuned input circuit, the initial amplitude, as the oscillations start to build up, corresponds to the amplitude of the superimposed RF signal. The oscillations, therefore, reach their equilibrium more quickly because of the larger initial amplitude. The quenching frequency can be simply adjusted by altering the value of an RC circuit connected to the base of the detector transistor. The RF carrier is amplitude modulated with a desired information signal which is recovered at the output of the detector with a low pass filter which rejects the RF carrier and the quench signal.
This type of circuit is valuable because it is simple and economical to construct and it suppresses noise pulses of short duration because in superregeneration, the system is sensitive to the incoming signal for only a small fraction of the total time. It is desirable to use such circuits in components such as receivers for garage door openers, portable telephones and the like because of their small size, ease of construction and economy of construction.
However, they have some severe disadvantages. In the first place they are normally LC controlled. That is, they utilize inductors and capacitors in the feed back circuits which causes the oscillator to be unstable over time and to drift from its desired frequency of operation. In addition, any LC circuit is temperature sensitive which causes the frequency of operation to change with temperature change. Further, superregenerative receivers in the prior art have relatively wide reception bands and are therefore influenced by stray signals and noise which they can pick up and detect.
To overcome these problems, it is necessary to use a feed back circuit which has a low loss, is temperature stable, and provides the necessary phase shift from the output to the input to cause oscillation of the detector.
A surface acoustic wave delay line device is very temperature stable and would be ideal to place in the feedback circuit as a phase shift element except that the prior art surface acoustic wave delay lines have an insertion loss which is so high that sufficient feedback cannot occur with a single transistor oscillator.
Prior art resonators which are formed of SAW devices have low insertion loss but have a Q that is so high that the quench circuit cannot function properly. Thus to construct a superregenerative detector with prior art SAW devices would prohibit the construction of a superregenerative detector with a single transistor and would have to be quite complex using a separate quench oscillator and other circuits which would negate simplicity and economy of construction.
The present invention relates to a superregenerative receiver which does in fact utilize a surface acoustic wave device in the feed back circuit which is not only a delay line but is also a low loss device. Thus, it allows the construction of a superregenerative detector with one transistor, is temperature stable, is not LC controlled, does not drift in frequency, and has a very narrow reception band therefore eliminating the reception of noise and stray signals which are close to the operating frequency of the detector. The ideal SAW device to be used is formed as a single phase unidirectional transducer with quarter wave length electrodes and spacing thus allowing operation of the detector at very high frequencies.
Thus it is an object of the present invention to provide a superregenerative detector including an oscillator having an output and an input and a surface acoustic wave device forming a feed back loop for coupling the output to the input to cause oscillations.
It is also an object of the present invention to couple a modulated RF signal to the oscillator input and a low pass filter to the output to recover the modulation signal.
It is still another object of the present invention to provide a superregenerative detector wherein the surface acoustic wave device which forms the feed back loop for coupling the output to the input to cause oscillations is a low loss device and a delay line.
It is yet another object of the present invention to provide a superregenerative detector which has a surface acoustic wave device forming a feed back loop for coupling the output to the input to cause oscillations and wherein the SAW device is a single phase unidirectional transducer formed with .lambda./4 electrodes.